R. Cutler, LCMB, NIA, has suggested that a possible mechanism of aging may be "dysdifferentiation": the loss of control of gene expression by aging cells, so that genes become aberrantly expressed in old age. This idea was based on his observation that globin message was present in liver and brain cells of mice, and that this level increased with age. Recent findings have provided additional evidence that this idea may be relevant to aging: it has been shown that methylation of DNA (associated with the shutdown of gene expression) decreases with age and that genes on Lyonized X-chromosomes frequently become active in old age. We have chosen five genes, each specific to a different tissue, and asked whether any of them is expressed in any other of the tissues in old age. No such change was found at the one copy per cell level.